This invention relates to electrical circuit switching devices, particularly but not exclusively for contactors, relays and the like.
The switching of the circuits of electric motors and other loads is commonly performed by switching devices such as electromagnetic contactors, electromagnetic relays etc. A common problem in such devices is contact fouling. Measures are therefore usually taken to reduce fouling, in particular by providing for a mechanical wiping action of the contacts during operation of the switching device for switching the load circuit.
One conventional switching device is illustrated by FIG. 13 of the accompanying drawings, which shows an exploded view in perspective of a contactor.
The illustrated device has a moulded plastics base 10 onto which is fastened a moulded plastics body 16. The body is attached to the base in any convenient way, for example by means of screws. A stationary core 14 and an associated energizing coil 12 are seated in the base 10.
A movable cross bar 18 is disposed in the body 16 and can slide in the body towards and away from the base 10. A U-shaped movable core 20 is disposed inside the body, with its limbs facing the ends of the stationary core 14. The cross piece of the movable core 20 is located in a recess in the cross bar 18, by means of a spring clip 22 which extends through the movable core and of which the ends engage recesses in the cross bar. The core 20 and cross bar 18 therefore move with one another towards or away from the stationary core 14 according to whether the coil 12 is energizing or not.
One or more springs 24 seated between the cross bar 18 and the base 10 urge the cross bar away from the base, so that normally, when the coil 12 is not energized, the movable core 20 and cross bar 18 are held away from the stationary core 14.
When the coil 12 is energized, the movable core 20 is pulled towards the stationary core 14, thereby moving the cross bar 18 towards the base 10.
The body 16 is provided with stationary contacts connected to terminals for connection of external cables, and the cross bar 18 is provided with corresponding contacts, so that the movement of the cross bar opens and closes the switch.
In the illustrated example, a stationary contact member 28 with a stationary contact 26 is fixed to the body 16.
The cross bar 18 is provided with a contact bar 32 which is mounted so that it can slide in an opening 18a provided in the cross bar 18, and a spring 30 urges the contact bar 32 towards one end or bottom surface 33 of the opening 18a, namely the end or bottom surface 33 closer to the base 10 (see FIG. 11). On the side of the contact bar 32 that faces the base 10, contacts 34 are provided at both ends of the contact bar 32. The contacts 34 face respective contacts 26. Therefore, if the cross bar 18 and with it, the contact bar 32, are moved by the core 20 towards the base 10, the movable contacts 34 meet the stationary contacts 26, and the switch is closed. When the coil 12 is de-energized, springs 24 move the cross bar 18 away from the base 10, carrying the contacts 34 out of contact with the contacts 26. The spring 30 allows the contact bar 32 to `float` and provides contact pressure to ensure good contact between the contacts 34, 26.
A removable arc cover 36 is fitted on the body 16 to constrain the arc formed when the contacts make or break contact, and to prevent damage due to arc heat.
In this device, an oxidized layer may form on the mating surfaces of the contact 26, 34, and/or dust may adhere to the contact surfaces. During switching, the movable contact bar 32 always remain parallel to its initial position and moves in a direction perpendicular to its length, so that the contacts 34 only move directly toward and away from the contacts 26. Thus there is no wiping action at the contact faces, and any oxidized layer or dust remain on the contacts. The performance of the contacts therefore deteriorates because the contact resistance between the contacts 26, 34 increases due to the oxidized layer and/or dust, and arcing may be enhanced, with pitting of the contacts. Operational reliability therefore deteriorates.
To induce a wiping action for cleaning the contact surfaces, FIG. 15 shows a projection 37 disposed on one side of the contact bar 32, so that when the contact bar 32 is in contact with the surface 33 of the cross bar at the end of the opening 18a, the contact bar 32 tilts sideways about the projection 37. Consequently, when the cross bar is moved so as to bring the contacts 34 into initial contact with the contacts 26, the contact bar 32 will tilt about the projection 37, thereby rocking the contacts 34 on the contacts 26, so as to wipe the contact surfaces. The reverse sequence of events provides a wiping action also when the contacts 34 are moved away from the contacts 26.
However, this construction has the disadvantage that the raw material is not effectively used when the contact bar 32 is manufactured, and the contact bar is relatively weak. The contact bar and the cross bar wear out quickly, because the projection 37 presses and rocks on a very small area of the cross bar, and the projection 37 is sharp. The performance of the switch therefore deteriorates, albeit over a relatively long period in most cases.